When a call is placed to a 9-1-1 Public Safety Answering Point (PSAP) from a landline PSTN telephone, the PSAP is able to determine the caller's location based on the Caller ID information supplied with the call. This approach is not feasible when the caller is using a cellular telephone. Instead, the PSAP must rely on triangulation among the cell towers that have detected the phone (usually not adequately precise) or the automatic self-reporting of GPS coordinates by the phone (often not available, even with phones that have an inherent GPS capability due to line of sight limitations and the effects of building shielding).